This is expected in the sense that it tests that we don't support the rootNode attribute; we currently do support it, but it is preffed off in release builds. There's no need to land anything on trunk.
Bug 1269155 is expected to fix this once and for all.

What verb tense are you using to call 51 and 52 unaffected, alternate-future-with-different-past? Bug 1269155 hasn't been meaningfully touched since last June, 51 merges to beta on November 7th, and when it does this test will again fail.